Turbochargers are a type of forced induction system. They deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. A smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, will reduce the mass and aerodynamic frontal area of the vehicle.
Turbochargers use the exhaust flow from the engine exhaust manifold to drive a turbine wheel, which is located in the turbine housing. Once the exhaust gas has passed through the turbine wheel and the turbine wheel has extracted energy from the exhaust gas, the spent exhaust gas exits the turbine housing through the exducer and is ducted to the vehicle downpipe and usually to after-treatment devices such as catalytic converters, particulate traps, and NOx traps.
The basic turbocharger configuration is that of a fixed turbine housing. In this configuration, the shape and volume of the turbine housing volute is determined at the design stage and is cast in place. The basic fixed turbine housing is the most cost-effective option simply because it is the most simple and has the fewest parts.
The next level of sophistication is that of a wastegated turbine housing. In this configuration, the volute is cast in place, as in the fixed configuration above. The volute is fluidly connected to the exducer by a duct. Flow through the duct is controlled by a wastegate valve. Because the outlet of the wastegate duct is on the exducer side of the volute, which is downstream of the turbine wheel, flow through the wastegate duct, when in the bypass mode, bypasses the turbine wheel, thus not contributing to the power delivered to the turbine wheel. When a wastegated turbocharger is used, the wastegate arm part of the wastegate pivot shaft is connected to the wastegate valve on the inside of the turbine housing and to an actuator located external to the turbine housing. The wastegate pivot shaft extends between the turbine housing volute and the outside of the turbine housing, rotating in a cylindrical bearing, or directly in the turbine housing. Because a clearance exists between shaft and bearing bore, a flow of hot, toxic exhaust gas through this gap is possible.
The next level of sophistication in boost control of turbochargers is the VTG (the general term for variable turbine geometry). Some of these turbochargers have rotating vanes and some have sliding sections or rings. Some titles for these devices are: variable turbine geometry (VTG); variable geometry turbine (VGT); variable nozzle turbine (VNT); or simply variable geometry (VG).
VTG turbochargers utilize adjustable guide vanes (31) mounted so as to rotate between a pair of vane rings (30, 32) and/or one vane ring and a nozzle wall. These vanes are adjusted to control the exhaust gas backpressure and the turbocharger speed by modulating the exhaust gas flow to the turbine wheel. In many configurations the vane shaft (36), on which the vane rotates, is mechanically connected to a vane arm (33) situated above the upper vane ring. The vanes can be rotatably driven by forks (42) engaged in an adjusting ring (22). In many configurations, the forks on the ends of the vane arms drive independently rotatable “small turning blocks” (38) to minimize friction in the system and to deal with the inevitable distortion and corrosion in the turbine housing, and thus the linkages.
FIGS. 1A and 1B show a VTG configuration in which the adjusting ring (22) is supported by ramparts (34) on the vane arms (33). A large turning block (37) is connected by a shaft to the adjusting ring (22).
Displacement (by an actuator) of a control shaft (23) rotates the pivot arm (24) attached towards the outside end of a pivot shaft (29). Attached toward the inside end of the pivot shaft is a pivot shaft fork (35). The displacement of the control shaft (23) results in a rotation of the pivot shaft (29) about its axis (28). This rotation is carried inside the housing to translate into rotation of the pivot shaft fork (35). The rotation of the pivot shaft fork acts on the large turning block (37), which results in rotation of the adjusting ring (22) about the turbocharger centerline (1). The rotation of the adjusting ring (22) about the turbocharger centerline (1) causes the multiple small turning blocks (38) to rotate about the turbocharger center line (1) while each of the blocks is also free to rotate about the centerlines (27) of the vane shafts (36). This motion of the small blocks causes the vane arms (34) to rotate about the centerlines (27) of the vane shafts (36) and change the angle of attack of the vanes (31) to the exhaust flow.
Turbine housings experience great temperature flux. The outside of the turbine housing faces ambient air temperature while the volute surfaces contact exhaust gases ranging from 740° C. to 1050° C. depending on the fuel used in the engine. The complicated translated motions described above enable the actuator to control the flow to the turbine wheel in an accurate, repeatable, non jamming manner.
A VTG is used to control the flow of exhaust gas to the turbine wheel, and thus to drive the compressor to compress inlet air, as well as to control the turbine back pressure required to drive EGR exhaust gas, against a pressure gradient, into the compressor system to be re-admitted into the combustion chamber. The back pressure within the turbine system can be in the region of up to 500 kPa. This high pressure inside the turbine stage will result in escape of exhaust gas to atmosphere through any apertures or openings. Passage of exhaust gas through these apertures is usually accompanied by black soot residue on the exit side of the gas escape path. This soot deposit is unwanted from a cosmetic standpoint, and the escape of said exhaust gas containing CO, CO2, and other toxic chemicals is a health hazard to the occupants of the vehicle, which makes exhaust leaks a particularly sensitive concern in vehicles such as ambulances and buses. From an emissions standpoint, the gases which escape from the turbine stage are not captured and treated by the engine/vehicle aftertreatment systems.
A typical method for minimizing the flow of exhaust gas through the aperture formed by a shaft rotating within a cylindrical bore is the use of a piston or seal ring. Piston rings are commonly used within a turbocharger to control the passage of oil and gas from the bearing housing to both compressor and turbine stages and vice versa. BorgWarner has had piston rings for this purpose in production since at least 1954 when the first mass production turbochargers were produced. For a slowly rotating shaft (as slow as 150 RPM, as compared to >150,000 RPM for the turbocharger rotating assembly), the same method and design is typically employed since the piston rings are in general inventory and function well as a gas passage inhibiting device.
In “slowly rotating shaft” usages such as those transmitting actuator driven VTG commands to rotate vanes or wastegate actuators commanding opening of wastegate valves, there often exist non-rotational forces twisting, rocking, or skewing these shafts. These motions can cause premature wear in the piston ring or it's mating grooves, and, at worst, can cause locking of the rotation or failure of the piston ring as it pinches in its grooves. These situations exacerbate the leakage of gases and particulate from the turbocharger to the exterior atmosphere.
Thus it can be seen that there is a need for a relatively simple, cost-effective design to enhance the seal-ability and life of the gas seal used for “slowly rotating” VTG and wastegate pivot shafts in turbochargers.